This invention relates to a thick film fuse assembly for high reliability applications. These fuses are particularly suitable for high voltage, high amperage circuits which may be operated in high vacuum environments, in which a very high degree of reliability is required. Additionally, these fuses are suitable for use in environments which may subject the fuse to relatively high levels of mechanical shock and vibration. A typical application for this type of fuse is the fusing of satellite power systems.
Thick film high reliability fuses have, in the past, been constructed with a single thick film element of conductive metal printed on a thermally insulative substrate with thick film terminations which are used to provide electrical contact with the thick film fuse element. In this context, xe2x80x9cthick filmxe2x80x9d refers to the process of screen printing and firing electrical components on a substrate, not to the actual thickness of the components. In many cases the elements are quite thin i.e. several tenths of a micron. In the screen printing process the fuse components are patterned and printed on the substrate, the firing process of approximately one hour is used to remove the solvents and bind the components to the substrate. The fuse element is covered with a layer of arc suppressant glass which has a relatively low (450xc2x0 C.) melting point. Leads are connected to the terminations and the entire package is encapsulated by an insert molding operation utilizing a high temperature thermoplastic or thermoset plastic with low outgassing characteristics.
Traditional thick film fuse assemblies (constructed with gold elements) clear (blow) in the following manner: excessive current in the fuse heats the fuse element to 450xc2x0 C. which is the melting temperature of the arc suppressant glass. When the arc suppressant glass melts, the thermal equilibrium of the fuse is altered. The fuse element goes into thermal runaway which allows the element to melt at temperatures at or above 1050xc2x0 C. The melted fuse element migrates into the arc suppressant glass located above it, which prevents a continued arcing process. These fuses have a limitation in that the maximum operating voltage is approximately 72 volts D.C. for fuses rated above 1 or 2 amps. However, newer satellite power systems operate above 100 volts D.C. at well above 5 amperes which renders traditional thick film fuse constructions unusable.
The reason for the voltage limitation of traditional thick film fuses is that during the overload clearing action the fuse element material (throat region) must be completely absorbed by the arc suppressant glass to prevent arcing and restriking which could result in a catastrophic failure, such as the failure of a fuse to completely open or a breaching of the fuse package. In traditional thick film fuse constructions the fuse element thickness is increased as the fuse amperage rating is increased. Thus more fuse element material must migrate into the arc suppressant glass when a 5 amp fuse is cleared than when a 1 amp fuse is cleared. At voltage levels above 72 volts D.C. the arc suppressant glass cannot reliably suppress arcing and restriking at fuse ratings greater than 1 or 2 amperes. It is believed that the larger mass of fuse element material which must migrate during clearing saturates the arc suppressant glass and decreases its ability to suppress the arc, which can promote catastrophic failure.
In the first construction of a fuse element in accordance with a present invention the fuse element consists of an insulative substrate in which a plurality of low mass thick film fuse elements are disposed in parallel on the substrate. Thick film contact pads electrically connect to the fuse elements to permit attachment of lead wires and a layer of low melting point arc suppressant material covers the fuse elements. This construction permits a higher voltage and current rating for the fuse element because the fusible element is not concentrated in one area. Thus, there is more arc suppressant glass to absorb the material of the element, which provides a more reliable fuse.
In the second embodiment of a fuse assembly in accordance with the invention the fusible elements comprise thick film, screen printed, end portions and gold wires which are positioned so as to stand above and away from the insulative substrate. This construction provides a faster initiation of the clearing action. The wire portion of the fuse element is completely surrounded by arc suppressant glass. During an overload clearing condition the arc suppressant material is better able to limit arcing and restriking because the material of the fusible element is not concentrated in one area as is the case with single element fuses. Finally, if during the clearing action the wire portion of the fuse should burn back to the thick film portion of the element the thick film portion will also migrate into the arc suppressant glass without breaching the fuse package.